GB858250A - Improvements in or relating to permanent magnet assemblies - Google Patents

Abstract

858,250. Loud-speakers. DARWINS Ltd. July 1, 1959 [July 1, 1958; July 23, 1958], Nos. 20992/58 and 23628/58. Class 40 (4). [Also in Group XXXV] A permanent magnet assembly for operating a loud-speaker coil moving through a gap 18 in magnet pole plate 13 comprises a mild steel centre polepiece 14 with a cylindrical slot 19 in it, below the gap, to form a ring 20 of magnetic material to carry the leakage flux to the undersurface of the pole plate. The centre pole 14 is surrounded by permanent magnet 11 to which pole plates 12, 13 are attached. A cylinder 16 extends through hole 17 in plate 13 to define the gap 18. The ring 20 whose external surface may be conical (Fig. 10, not shown). may be integral with the polepiece or may be a separate piece resting on a shoulder of the pole (Figs 2, 3, not shown). In another modification a mild steel centre pole is surrounded by a shorter cylinder forming the ring and resting on the bottom pole plate (Figs. 4, 5, not shown). In a further modification the centre pole is of permanent magnet material secured to the bottom of a surrounding pot of mild steel and has a shoulder to take a mild steel or permanent magnet ring as above (Fig. 6, not shown). Alternatively the permanent magnet has a mild steel polepiece which may have the ring integral with it (Fig. 7, not shown) or as a collar resting on a shoulder of the permanent magnet (Fig. 8, not shown). Another modification has a cylindrical central pole surrounded by a shorter cylinder of permanent magnet material (Fig. 9, not shown).